A wideband or UWB-IR (ultra wide band Impulse Radio) transmission uses the whole of the band of the channel, thereby requiring the use of time division duplexing (TDD) to exchange data between two nodes. As a result, naturally, the access scheme used for this kind of transmission is time division multiple access (TDMA, AMRT in French).
Moreover, it is known that systems using TDMA are limited in capacity and the product of the number of users times the useful throughput per user is constant.
A way to increase the bandwidth of such a network is, for example, to combine a code division multiple access (CDMA) technique with TDMA which then allows several users to transmit during one and the same time horizon. CDMA in UWB-IR is made possible by the use of time-hopping codes (THC).
Like all code division multiple access CDMA systems, UWB-IR-THC (use of time-hopping codes) suffers from the phenomenon of interference caused by other users (MUI: Multi-User Interference).
In a general way, several schemes are potentially possible for reducing or canceling the phenomenon of multi-user interference or MUI. When restricting oneself to low-cost, low-consumption implementation solutions, one is then reduced to analog linear receivers of “rake” type where the decision is taken on a linear combination of delayed versions of the signal received. The only relevant parameter left in the design of the system for performance optimization is the datum of the time-hopping code. It is then possible to find a criterion which allows minimization of the output variance of the MUI at the output of the receiver and therefore enables the best performance to be ensured.
The transmission system's physical layer proposed according to the invention is, notably, based on the UWB-IR ultra wideband impulse technology.
The principle of UWB-IR is to transmit the information by means of very short impulses (of the order of a nanosecond) in baseband.
UWB-IR-THC transmission systems have essentially been described at the level of the physical layer, for example in document [1]. The idea of the THC is to transmit the impulses by spacing them out temporally in a pseudo-random manner, each user having his own sequence. In principle this allows use of UWB in an asynchronous multi-user context. The use of this physical layer in practical systems is described for example in the two articles [2] and [3].
However, no prior art reference describes the way to achieve a multiple access system based on the use of time-hopping codes.
To the knowledge of the applicant, the only solutions proposed up till now for constructing the codes consist in applying the results of constructing codes dedicated to different transmission systems (essentially dedicated to frequency hopping). An example is given in document [4]. These codes fall into the class of congruent codes, but do not comply with the criterion for minimizing the variance of the MUI considered here.
More recently, work published in [5] shows how to characterize the best codes (within the sense of the minimum variance of the MUI) for a UWB-IR transmission using amplitude modulation (PAM: Pulse Amplitude Modulation).